This invention relates to control switches for electrical devices. In particular, it relates to devices for controlling electrical equipment from an area that contains or may contain explosive fumes or vapors, dust, or the like.
Control of electrical equipment from areas that contain explosive vapors presents some technical problems that have been solved in various ways. A hazard arises because the opening of electrical contacts carrying current in a circuit that has any inductance will produce an arc that may ignite the explosive vapors. One approach that has been taken in the past to prevent this is the use of an explosion-proof enclosure. This is an enclosure that is made to be sealed so as to be water-tight and pressure-tight. Enclosures for electrical devices to be used in such an atmosphere are typically cast instead of stamped. They are normally assembled with gaskets and bolts to maintain a seal. The result is an enclosure that is considerably more expensive to purchase than those made for areas that are not hazardous. The hazardous-duty enclosures require maintenance not required by enclosures designed for non-hazardous areas.
One approach to the problem of electrical control in hazardous areas has been the so-called inherently safe switch. This has been done using optical fibers as a means for linking mechanical controls in the hazardous area to electrical controls outside the hazardous area. In such a system, a source of light outside the hazardous area is coupled to an optical fiber that is taken to an input device in the hazardous area. The input device, which may be a pushbutton, foot switch, limit switch, pressure switch or the like, includes an interruptible path for light which may be reflected to a second fiber in the input device that is connected back to a receiver outside the hazardous area. Operation of the switch in the hazardous area either permits the passage of light or interrupts it. Systems such as the one described have been used in petrochemical processing plants, refineries, plants manufacturing explosives, grain-handling facilities and the like.
The inherently safe systems now in use present several disadvantages. The first of these is the frequent use of two optical fibers for each switch. One optical fiber is used to conduct light from the light source to the input device, and a second is used to conduct reflected light back to a receiver outside the hazardous area. The light source and receiver are often located together so that a double optical cable may be used. However, the use of two such cables often requires four cable terminations that must be made to connect an inherently safe switch.
Another disadvantage that must be overcome in an optical switch is spurious responses to ambient light. A light-operated switch should respond only to a signal from a desired source, and should not be triggered by changes in the ambient light level.
It is also useful if an optical switch for inherently-safe use is made to be installed by electricians without special tools or training. If an optical switch requires field installation of terminations for which the ends of an optical cable must be ground and set with epoxy or the like, the switch is made much more difficult to use.